Perennials that won’t tolerate leaf mulches

In a recent post, Evelyn Hadden shared some very useful tips on how fall’s leaves can be used in the garden. As a perennial enthusiast, I’d like to add a couple of caveats – a mulch of autumn leaves can be fatal to certain kinds of perennials.

A mulch of freshly fallen leaves applied an inch or two thick, or even just a heavy leaf fall from nearby trees, tends to keep the ground beneath it damp, especially if the leaves are large and you don’t shred them before applying them (I always recommend shredding leaves with a dedicated leaf shredder or a lawn mower when using them as mulch).

Because they keep the ground damp, leaf mulches of any kind, shredded or otherwise, are not beneficial for silvery, woolly-leaved plants such as lamb’s ears (Stachys byzantina) or lavenders (Lavandula spp.). These plants are adapted to dry sites — their silver hue and hairy surface are adaptions to protect them against dehydration and drought – and they will rot if kept consistently damp.

Gray, hairy plants like this lamb’s ear won’t tolerate leaf mulches

Other perennials that won’t tolerate prolonged dampness include many culinary herbs such as thyme, oregano, and sage, all of which are native to the dry, rocky soils found around the Mediterranean. In fact, Mediterranean plants as a whole generally do not flourish when swaddled with leaves.

Succulents likewise will rot if kept damp; keep leaf mulches away from your sedums. Alpine plants are also vulnerable to damp, especially in wintertime – do not use leaf mulches in the rock garden (a gravel mulch is far better there).

Finally, as Dale Hendricks emphasized in a recent email, leaf mulches are also problematic for herbaceous evergreens such as heucheras and hellebores. If the mulch is applied simply by raking or blowing leaves onto the garden bed, then it is likely to bury the perennials’ foliage and interfere with their wintertime photosynthesis (a heavy leaf fall from nearby trees can achieve the same thing if left undisturbed). When used around evergreens, I recommend shredding the leaves thoroughly and then tucking the mulch in by hand so as not to bury the foliage.

Care must be used in mulching evergreens like these coral bells

Photos by Susan Harris.

Posted by

Thomas Christopher
on November 28, 2015 at 11:54 am, in the category Shut Up and Dig.

I don’t favor bark mulches for perennials — as it decomposes, the bark temporarily depletes the nitrates in the soil and increases the need for fertilization. Both bulbs and milkweed typically respond well to mulching.

Thank you. I will clear my leaves from those plants. Don’t want to kill anyone in the gorgeous ground. I think the leaves are most beneficial in building up new beds or garden or annual beds. I have a space in the back corner of my yard where I dump left over leaves and cover sporadically with shovel fulls if dirt or fine bark. It gives me extra dirt for pots etc.

I live in the Sonoran Desert, and I’m pruning back lantana. It is susceptible to freezes. I was going to cover root base and short branches with a “gorilla” mulch. Any recommendations or advice? Thank you.

I’m not familiar with “gorilla mulch” but I certainly wouldn’t worry about persistent damp in your location, unless you water excessively. Any loose, airy mulch (including shredded leaves) should provide some protection against short-term dips in the temperature.

We generally recommend opting for a chopped leaf mulch instead of those thick bark colored mulches in garden beds as an economical alternative, but this article brings up a lot of really great points. In my personal gardens, I notice that evergreen perennial plants do well with a carefully placed mulch of fallen pine needles too. One rule to always remember on most all plants that you mulch is to make sure you’re keeping the mulch away from the bases of the plants, so the mulch doesn’t rot crowns- in many cases, not all. But it’s generally a good idea to get into the habit of. You should never heavily broadcast mulch without taking the time to get your hands into the garden and move the mulch in ways that are helpful.